1. Field of the Invention
The present invention relates generally to apparatus and methods for controlling a homogeneous charge compression-ignited internal-combustion engine. More particularly, the invention concerns a control apparatus and method for a homogeneous charge compression-ignited internal-combustion engine, suitable for combustion mode switching from spark-ignited combustion to homogeneous charge compression-ignited combustion.
2. Description of the Related Art
Homogeneous charge compression-ignited gasoline engines that achieve compression-ignited combustion by compressing an air-fuel mixture to simultaneously improve both fuel efficiency and gas emissions reduction performance are catching attention as the engines used for automobiles and the like. Compared with the spark-ignited combustion type of engine that ignites and burns the mixture by generating a spark from an ignition plug, the homogeneous charge compression-ignited type of engine reduces fuel consumption by adopting a high compression ratio for enhanced efficiency, reducing a pump loss ratio, and applying rapid combustion for minimum cooling losses, and reduces the concentration of NOx in the gas emissions by burning the mixture at low temperature. The latter engine can therefore improve fuel efficiency and gas emissions reduction at the same time.
Realizing a driver-requested engine torque by executing both spark-ignited combustion and homogeneous charge compression-ignited combustion and switching combustion mode control to one of the two combustion modes, depending on particular conditions, has been proposed during the application of a homogeneous charge compression-ignited engine to an automobile.
Introduction of internal EGR (Exhaust Gas Recirculation) is among the possible methods of implementing homogeneous charge compression-ignited combustion. Spark-ignited combustion uses a relatively rich air-fuel mixture and can therefore be implemented in a region relatively low in internal EGR ratio. Conversely, homogeneous charge compression-ignited combustion based on internal EGR employs a relatively lean air-fuel mixture and can therefore be implemented in a region relatively high in internal EGR ratio. In addition, an unstable combustion region in which both forms of combustion become unstable is present between the above two regions.
Engine parameters that govern combustion are modified when the combustion mode of a homogeneous charge compression-ignited engine is switched from spark-ignited combustion to homogeneous charge compression-ignited combustion. These engine parameters include, for example, the amount of fuel injected from an injector, fuel injection timing in which fuel injection from the injector is started, ignition timing in which the release of a spark from an ignition plug is started, the opening angle of a throttle valve capable of controlling the amount of air flowing into a combustion chamber, the opening/closing control timing or valve lift of an air intake valve or exhaust valve capable of controlling the flow of the mixture into the combustion chamber, and the opening angle of a swirl control valve capable of controlling the flow intensity of the mixture within the combustion chamber. Since an actuator for controlling the engine parameters has definite response characteristics, there have been problems in that during mode switching, misfiring or fluctuations in torque are caused by the occurrence of an in-cylinder state substantially equivalent to that of the engine in the unstable combustion region mentioned above.
Among known methods for solving these problems is a method in which a period for setting up the foregoing engine parameters as transient ones is provided during mode switching from spark-ignited combustion to homogeneous charge compression-ignited combustion to provide against the unstable combustion region. JP-A-11-336600, for example, discloses such a method. According to JP-A-11-336600, combustion is stabilized in the transient parameter setup period by advancing ignition timing to reduce torque fluctuations at a mode switching operation.